Think of it like pen and paper role playing without a printed character sheet, GM screen with all of the rules specific stuff on it, Printed NPC records specific to a game etc..
CoreRPG allows you to:
- have story entries the GM can share with the players, have links to other story entries, text ready to add to the chat window as a description or NPC speech, etc..
- have map images with grids, mask (fog of war), tokens (visible and invisible to the players), etc.
- image handouts you can share with one or more of the players.
- a free form character sheet for basic info (there's also an extension that expands on the character sheet) which includes attributes with modifiers, inventory and notes page.
- a basic combat tracker for keeping track of the order of acting, etc..
- character portraits and tokens.
- free form NPC sheet where you can enter rolls of the form <dice>+<modifier>, e.g. Sword attack 1d20+3, and this will allow the GM to double click on the entry to auto roll 1d20+3.
- encounter entries with token pre-placement.
- free form item sheet including item identification.
- item/treasure parcels and auto assignment to players via the party sheet.
- tables for random generation of all sorts of info.
- calendar
- free form effects.
- notes for players and GMs
- library functionality.
- the ability to put most of the above into modules for future re- use.
- tower dice rolling (GM sees the roll, players don't).
- roll modifiers
As I said, basically all of the FG functionality but without the game rules specific details and automation. CoreRPG should allow a GM to run most games just like they were sitting at the table with their players - they have to ask the players to roll for something, the player rolls (everyone can see the rolls unless they are in the tower) adds any modifiers and tells the GM the result (or they could use the modifiers box to add modifiers before the roll), the GM determines the result of the roll and acts on it. That should be a familiar process to all people who have played RPGs face to face around a table top - FG is providing the virtual table top around which to play. CoreRPG also provides a lot of the extra non game rules specific features that make GMing easier (as listed above) and the player experience better.
Load up a test campaign with CoreRPG and have a play around... :)

